Recently, a transmission system such as a communication system, which transmits call data including image data and audio data between transmission terminals such as communication terminals, has been common with the demand to reduce expense and time for business trips. The spread of broadband environments now enables transmission and reception of high-quality image data or audio data, making it easy to grasp the situation of the other side of caller, which enhances the quality of communication.
For instance, in a mobile phone transmission system, when communication terminals start a call, a call request communication terminal requests a call destination communication terminal to transmit call start request data and further requests a communication management device to start calling. The communication management device manages these communication terminals, by management data session. The communication management device, having received the call start request data, requests a relaying device that relays the call data to connect a communication network between these communication terminals. When this communication network is connected between communication terminals via the relaying device, a call data session for transmitting the call data is started. When terminating the call, either one of the communication terminals terminates the call data session, and transmits call end data to the communication management device through the management data session. In this way, the communication management device receives the call start request data or the call end data, while associating this data with time information. In this manner, the communication management device is able to manage duration time of the call.
However, the management data session may be interrupted after the call data session is started between communication terminals due to a failure caused in a network or the communication management device. In such case, the call data session may be continued as long as the communication network between communication terminals is connected. However, the communication terminal does not transmit the call end data to the communication management device through the management data session.
Japanese Patent Application Publication No. 2000-112706 describes a log management system, in which log data regarding a printing job generated by a first computer is managed by a second computer. The first computer judges the operating state of the second computer, and operates to transmit the log data to the system.